Browsing by Subject "Microbical fuel cell"

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Treatment of benzene and ammonium contaminated groundwater using microbial electrochemical technology and constructed wetlands
(2018) Wei, Manman; Seifert, Jana
With the rapid development of modern industry, energy shortage and environmental pollution are getting more and more serious. Groundwater pollution is one of the most important problems. A multitude of remediation techniques in situ or ex situ have been used to treat contaminated groundwater. This thesis was to investigate whether groundwater contaminated mainly by benzene and ammonium can be remediated by constructed wetlands in combination with microbial electrochemical technology. The objectives of this thesis are (i) to develop and test systems for removing pollutants and simultaneously recovering energy from contaminated groundwater, (ii) to maximize the benefits of both constructed wetland and microbial electrochemical technology while treating contaminated groundwater, (iii) to elucidate the underlying electrochemical reactions and pollutant degradation pathways, and (iv) to investigate microbial active species and functional proteins involved in benzene degradation and ammonium removal. A microbial fuel cell (MFC) equipped with an aerated cathode and a control without aeration at the cathode were designed to remove benzene and ammonium from contaminated groundwater collected in the Leuna site (Saxony-Anhalt, Germany). The performance of pollutant removal and electricity generation was investigated and compared in the two reactors. Electrochemical processes occurring in the MFC were determined by benzene and ammonium spiking experiments as well as oxygen interruption experiments. Additionally, the biodegradation pathways and dominant organisms were elucidated by compound specific stable isotope analysis (CSIA) and Illumina sequencing. The results indicated the principal feasibility of treating benzene and ammonium contaminated groundwater by a MFC equipped with an aerated cathode. Benzene (~15 mg/L) was completely removed in the MFC, of which 80% disappeared already at the anoxic anode. Ammonium (~20 mg/L) was oxidized to nitrate at the cathode; this reaction was not directly linked to electricity generation. The maximum power density was 316 mW/m3 net anoxic compartment (NAC) at a current density of 0.99 A/m3 NAC. Coulombic and energy efficiencies of 14% and 4% were obtained based on the anodic benzene degradation. Benzene was initially activated by enzymatic monohydroxylation at the oxygen-limited anode; the further anaerobic oxidation of the intermediate metabolites released electrons accompanied by electrons transfer to the anode. Dominant phylotypes at the MFC anode revealed by 16S rRNA Illumina sequencing were affiliated to the Chlorobiaceae, Rhodocyclaceae and Comamonadaceae, presumably associated with benzene degradation. Nitrification took place at the aerated cathode of the MFC and was catalyzed by phylotypes belonging to the Nitrosomonadales and Nitrospirales. The control reactor failed to generate electricity, although phylotypes affiliated to the Chlorobiaceae, Rhodocyclaceae and Comamonadaceae were dominant as well; the control reactor can be thus regarded as a mesocosm in which granular graphite was colonized by benzene degraders, but showed a lower benzene removal efficiency compared to the MFC. In order to enhance benzene and ammonium removal while simultaneously harvesting energy, a constructed wetland integrated with microbial electrochemical technology (MET-CW) was established by embedding four anode modules into the sand bed and connecting it to a cathode placed in the open pond inside a bench-scale horizontal subsurface flow constructed wetland (HSSF-CW). Compared with the control CW, enhanced benzene and ammonium removal efficiencies were found in the MET-CW. The electrochemical performances of anode modules located at the four different depths were compared; the results showed that anode modules located in the deep layer (Module 3 and 4) had the relatively high power densities whereas the power densities located in the upper layer (modules 1 and 2) were extremely low. The initial activity mechanism of benzene degradation was analyzed by CSIA. Ammonium removal processes were assessed using nitrogen isotope fractionation of ammonium. Functional proteins and active microbial species involved in nitrogen transformation processes were detected using protein-based stable isotope probing (protein-SIP) with in situ feeding of 15N-NH4+. Additionally, potential denitrification and anammox rates were measured using Nitrogen isotope tracing. The results demonstrated that benzene and ammonium removal in a CW can be improved by combination with microbial electrochemical technology. The enhanced benzene removal was linked to the use of the anode modules as electron acceptor, whereas efficient ammonium removal was probably attributed to the elimination of inhibition effects by the co-contaminant benzene. Benzene was initially activated by monohydroxylation, forming intermediates which were subsequently oxidized accompanied by extracellular electron transfer, leading to current production. Partial nitrification accompanied by either heterotrophic denitrification or nitrifier-denitrification was mainly responsible for NH4+-N removal in the MET-CW, whereas anammox played a minor role. However, the contribution of anammox was markedly increased at the location near to the anode modules. In summary, this research indicated that microbial electrochemical technology can be used to improve the performance of pollutant removal while simultaneously harvesting energy from contaminated groundwater. Especially, the combination of MET with other traditional treatment approaches (e.g. constructed wetland) is a promising alternative to treat contaminated water.